Cities are thought to be refuges for wild bees, since recent studies have documented increased bee species richness in urban centres. However, urban gradients may present a series of environmental filters that decouple phylogenetic or functional diversity from species richness. Specifically, regions with high amounts of urbanized area (e.g. paved) or high habitat heterogeneity may lead to a non-random subset of closely-related bee species that share similar functional traits. Therefore, understanding patterns of wild bee diversity across urban landscapes may shed light on community assembly and aid the maintenance of bee diversity and pollination services.
To assess the potential influence of urbanization on bee diversity and assembly, we estimated bee phylogenetic relationships from a molecular phylogeny and trait-based functional similarities using six bee functional traits. Wild bees were sampled using trap nests for cavity-nesting wild bees set up at 200 locations from 2011-2013 in Canada’s largest city, Toronto, Ontario. Species richness and standardized effect sizes of both mean-pairwise functional and phylogenetic distances were calculated for each community across multiple spatial scales (250m2, 500m2) along a habitat heterogeneity and urbanization gradient. Here, habitat heterogeneity and urbanization were characterized as edge density and proportion of urban area, respectively.
In total, 35 species were recorded across all locations from 2011-2013. Our study revealed that species richness increased with habitat heterogeneity at both 250m2 (p < 0.05) and 500m2 (p <0.05) scales, but not with urbanization (p > 0.05). At the 500m2 scale, phylogenetic diversity (p < 0.05) increased with urbanization, and both functional (p < 0.05) and phylogenetic diversity (p < 0.05) had a positive relationship with increasing urbanization at the smaller 250m2 scale. Habitat heterogeneity had no significant effect on either functional or phylogenetic diversity (p > 0.05).
Our results show that community assembly patterns of cavity-nesting bee species are influenced by urbanization. Both functional and phylogenetic diversity increased with urbanization, reflecting either changes in competitive hierarchies or increased resource availability in these habitats through local-scale human facilitation (e.g. exotic flowers in residential yards). Moreover, our study emphasizes that phylogenetic and functional approaches offer new insight into the relationship between urbanization and wild bee biodiversity compared to previous studies of species richness. Overall, these results suggest hope for the maintenance of pollination services in cities by providing evidence that urban habitats can harbour more functionally and phylogenetically diverse wild bee communities.